Stability analysis of arbitrarily shaped moderately thick viscoelastic plates using Laplace–Carson transformation and a simple hp cloud method

In this paper, the stability analysis of moderately thick time-dependent viscoelastic plates with various shapes is studied using the Laplace–Carson transformation and simple hp cloud meshless method. The shear effect of the plate is described by the first order shear deformation theory. The mechanical properties of the materials are supposed to be linear viscoelastic based on the constant bulk modulus. The displacement field is assumed to be the product of two functions, one being a function of geometrical parameters and the other a known exponential function of time. The simple hp cloud method is used for discretization which is based on Kronecker-delta properties. Thus, the essential boundary conditions can be imposed directly. A numerical investigation is made by employing the inverse of Laplace–Carson transformation. The time history of buckling coefficients of viscoelastic plates of various shapes with different boundary conditions is considered. Moreover, a number of numerical results are presented to study the effect of thickness, aspect ratio, different boundary conditions, and various shapes on the time history of buckling coefficients of the viscoelastic plate.     

Viscoelastic properties of mineralized alginate hydrogel beads

Alginate hydrogels have applications in biomedicine, ranging from delivery of cells and growth factors to wound management aids. However, they are mechanically soft and have shown little potential for the use in bone tissue engineering. Here, the viscoelastic properties of alginate hydrogel beads mineralized with calcium phosphate, both by a counter-diffusion (CD) and an enzymatic approach, are characterized by a micro-manipulation technique and mathematical modeling. Fabricated hydrogel materials have low mineral content (below 3?% of the total hydrogel mass, which corresponds to mineral content of up to 60?% of the dry mass) and low dry mass content (<5?%). For all samples compression and hold (relaxation after compression) data was collected and analyzed. The apparent Young's modulus of the mineralized beads was estimated by the Hertz model (compression data) and was shown to increase up to threefold upon mineralization. The enzymatically mineralized beads showed higher apparent Young's modulus compared to the ones mineralized by CD, even though the mineral content of the former was lower. Full compression-relaxation force-time profiles were analyzed using viscoelastic model. From this analysis, infinite and instantaneous Young's moduli were determined. Similarly, enzymatic mineralized beads, showed higher instantaneous and infinite Young's modulus, even if the degree of mineralization is lower then that achieved for CD method. This leads to the conclusion that both the degree of mineralization and the spatial distribution of mineral are important for the mechanical performance of the composite beads, which is in analogy to highly structured mineralized tissues found in many organisms.     

A novel hydrogel crosslinked hyaluronan with glycol chitosan

A novel hydrogel was prepared by crosslinking hyaluronan with glycol chitosan in aqueous solution using water soluble carbodiimide at nearly neutral pH and room temperature. The products can be easily formulated into injectable gels, various films, membranes and sponges for soft tissue augmentation, viscosupplementation, drug delivery, preventing adhesion of post operation, wound dressing and tissue engineering scaffolds. The said hydrogel has high water adsorption property and biostability. Rheololgical results of the gel showed a soft and viscoelastic structure. FTIR further confirmed the formation of amide bonds between carboxyl groups of hyaluronan and amine groups of glycol chitosan and no N-acylurea and other derivatives were identified.     

A novel optical coherence tomography-based micro-indentation technique for mechanical characterization of hydrogels.

Depth-sensing micro-indentation has been well recognized as a powerful tool for characterizing mechanical properties of solid materials due to its non-destructive approach. Based on the depth-sensing principle, we have developed a new indentation method combined with a high-resolution imaging technique, optical coherence tomography, which can accurately measure the deformation of hydrogels under a spherical indenter at constant force. The Hertz contact theory has been applied for quantitatively correlating the indentation force and the deformation with the mechanical properties of the materials. Young's moduli of hydrogels estimated by the new method are comparable with those measured by conventional depth-sensing micro-indentation. The advantages of this new method include its capability to characterize mechanical properties of bulk soft materials and amenability to perform creeping tests. More importantly, the measurement can be performed under sterile conditions allowing non-destructive, in situ and real-time investigations on the changes in mechanical properties of soft materials (e.g. hydrogel). This unique character can be applied for various biomechanical investigations such as monitoring reconstruction of engineered tissues.     

聚乙烯醇水凝胶的制备及性能

利用冷冻-解冻法制备聚乙烯醇(PVA)水凝胶,研究了不同因素对PVA水凝胶力学性能和溶胀特性的影响。结果表明,PVA水凝胶是一种典型的粘弹性材料,在一定应变区内,材料的拉伸模量随应变的增加而增大;拉伸强度和平均拉伸模量随PVA水溶液的浓度和冷冻-解冻循环次数的增加而增强,凝胶的最大拉伸强度和拉伸模量分别为2.27 MPa和0.95 MPa。溶胀特性研究显示,PVA水凝胶在生理盐水中的平衡溶胀比小于其在蒸馏水中的平衡溶胀比;凝胶的平衡溶胀比随浓度和冷冻-解冻次数的增加而下降,其下降趋势满足幂函数的变化规律;水凝胶的溶胀过程符合溶胀动力学方程。     

Modeling and characterization of viscoelasticity of PI/SiO2 nanocomposite films under constant and fatigue loading

In this paper, the viscoelastic behavior of PI/SiO₂ nanocomposite thin films under constant and fatigue tensile loading was studied. The cyclic hardening and viscous dissipation of PI/SiO₂ nanocomposite thin films during the fatigue process were experimentally investigated and analyzed by storage modulus, loss modulus and phase lag. The time-dependent deformation under constant and fatigue loading was simulated based on two viscoelastic models known as Burger model and Findley power law. Standard parameter analysis methodology was employed to interpret the structure–property relationship and deformation mechanisms of this kind of nanocomposites. In addition, the effects of nano-silica content, stress level and loading pattern (constant or fatigue loading) on the creep resistance of materials were discussed as well.     

Simultaneous Measurements of Geometric and Viscoelastic Properties of Hydrogel Microbeads Using Continuous‐Flow Microfluidics with Embedded Electrodes

Geometric and mechanical characterizations of hydrogel materials at the microscale are attracting increasing attention due to their importance in tissue engineering, regenerative medicine, and drug delivery applications. Contemporary approaches for measuring the these properties of hydrogel microbeads suffer from low‐throughput, complex system configuration, and measurement inaccuracy. In this work, a continuous‐flow device is developed to measure geometric and viscoelastic properties of hydrogel microbeads by flowing the microbeads through a tapered microchannel with an array of interdigitated microelectrodes patterned underneath the channel. The viscoelastic properties are derived from the trajectories of microbeads using a quasi‐linear viscoelastic model. The measurement is independent of the applied volumetric flow rate. The results show that the geometric and viscoelastic properties of Ca‐alginate hydrogel microbeads can be determined independently and simultaneously. The bulky high‐speed optical systems are eliminated, simplifying the system configuration and making it a truly miniaturized device. A throughput of up to 394 microbeads min^?1 is achieved. This study may provide a powerful tool for mechanical profiling of hydrogel microbeads to support their wide applications.     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.     

Nanoscale viscoelastic properties of an aligned collagen scaffold

Localised mechanical properties for aligned collagen scaffolds derived from Type 1 collagen were determined by application of nanoindentation based techniques. It was possible to measure the modulus and hardness with nanometre control over the depth of penetration and quasi-static testing under displacement control yielded average modulus values ranging from 1.71 GPa to 3.31 GPa; a narrower range of values than obtained by other methods. Hardness values of 222 MPa to 256 MPa were recorded and showed little scatter, highlighting the potential of nanoindentation hardness values as a reproducible and accurate measure of soft material properties. Open loop Load-displacement curves for the collagen exhibited the expected shapes for a viscoelastic material and it was thus possible to apply dynamic stiffness measurement at the nano scale. As well as determining the storage modulus (0.71 GPa) and the loss modulus (0.40 GPa) at the sub-micron length and nano depth resolution it was also possible to discriminate between surface and bulk readings allowing surface effects to be discarded if necessary. In addition to being a more accurate indentation method than atomic force microscopy, the localised dynamic mechanical properties of collagen were measured for the first time. These results demonstrate that this nanoindentation technique can serve as a powerful tool for the characterisation of collagen based biomaterials that are used as scaffolds for a variety of engineered tissues, such as artificial skin, skeletal muscle, heart valves and neuroregeneration guides.     

基于流变学的橡胶粉与基质沥青配伍性试验

基于流变学理论研究橡胶粉与不同来源基质沥青的配伍性,采用动态剪切流变仪（DSR）分别对不同基质沥青加工而成的橡胶沥青进行应变扫描、温度扫描、频率扫描等常规动态剪切流变试验,从相位角、复合模量和车辙因子等指标评价橡胶沥青黏弹特性,定性区分沥青四组分对橡胶沥青黏弹特性的影响,并对橡胶沥青进行滞回环试验,运用灰色关联数学分析方法定量给出沥青四组分对橡胶沥青的残余变形、弹性贮能、耗散能、弹性比例和复合弹性模量等指标的影响。结果表明：流变学理论是研究橡胶粉改性剂与基质沥青配伍性的有效方法;从能量角度评价沥青四组分对橡胶沥青黏弹特性指标的影响,沥青质对橡胶沥青残余应变影响较大;胶质组分对橡胶沥青弹性贮能和耗散能影响最大,而芳香分影响最小;沥青质组分对橡胶沥青弹性比例参数影响最大;芳香分含量可以提高橡胶沥青复合模量。     

蜂窝纸板静态弹性与黏弹性特性建模与参数识别

目的研究蜂窝纸板的静态力学行为。方法对蜂窝纸板试样进行压缩-恢复实验,用多项式表达试样的弹性力,用分数阶微分模拟试样的黏弹性力。考虑到试样变形和弹性力的对称性,根据压缩和恢复阶段的黏弹性力之差,利用信赖域方法识别试样的黏弹性参数,再进行试样的弹性特性参数识别。结果实验与模拟结果对比表明,提出的模型可准确模拟蜂窝纸板的力学行为,最大响应误差不超过7%。结论可利用三次多项式和五参数粘弹性模型分析蜂窝纸板的静态特性,为分析静态条件下蜂窝纸板的力学特性,研究蜂窝纸板的应力和应变的时变特性,优化包装设计方法提供了参考。     

Out‐of‐Plane 3D‐Printed Microfibers Improve the Shear Properties of Hydrogel Composites

One challenge in biofabrication is to fabricate a matrix that is soft enough to elicit optimal cell behavior while possessing the strength required to withstand the mechanical load that the matrix is subjected to once implanted in the body. Here, melt electrowriting (MEW) is used to direct‐write poly(ε‐caprolactone) fibers “out‐of‐plane” by design. These out‐of‐plane fibers are specifically intended to stabilize an existing structure and subsequently improve the shear modulus of hydrogel–fiber composites. The stabilizing fibers (diameter = 13.3 ± 0.3 µm) are sinusoidally direct‐written over an existing MEW wall‐like structure (330 µm height). The printed constructs are embedded in different hydrogels (5, 10, and 15 wt% polyacrylamide; 65% poly(2‐hydroxyethyl methacrylate) (pHEMA)) and a frequency sweep test (0.05–500 rad s^?1, 0.01% strain, n = 5) is performed to measure the complex shear modulus. For the rheological measurements, stabilizing fibers are deposited with a radial‐architecture prior to embedding to correspond to the direction of the stabilizing fibers with the loading of the rheometer. Stabilizing fibers increase the complex shear modulus irrespective of the percentage of gel or crosslinking density. The capacity of MEW to produce well‐defined out‐of‐plane fibers and the ability to increase the shear properties of fiber‐reinforced hydrogel composites are highlighted.     

Mechanical properties of nanohydroxyapatite reinforced poly(vinyl alcohol) gel composites as biomaterial

Nanohydroxyapatite reinforced poly(vinyl alcohol) gel (nano-HA/PVA gel) composites has been proposed as a promising biomaterial, especially used as an articular cartilage repair biomaterial. In this paper, nano-HA/PVA gel composites were prepared from mixing nano-HA particles modified by silicon coupling agent, with physiological saline solution (PSS) of PVA by freezing-thawing method. The effects of various factors on the mechanical properties of nano-HA/PVA gel composites were evaluated. It was shown that the mechanical behavior of nano-HA/PVA gel composites was similar to that of natural articular cartilage, which held special viscoelastic characteristics. The tensile strength and tensile modulus of the composites improved correspondingly with the increase of freezing-thawing times and concentration of PVA solution. The more concentration of PVA solution, the higher influence degree of concentration on the tensile strength of composites is. The tensile strength and tensile modulus of nano-HA/PVA hydrogel composites increased first and then decreased with the rising nano-HA content of the composites. The tensile modulus of the composites improved remarkably with the increase of elongation ratio.     

A novel collagen gel-based measurement technique for quantitation of cell contraction force

Cell contraction force plays an important role in wound healing, inflammation, angiogenesis and metastasis. This study describes a novel method to quantify single cell contraction force in vitro using human aortic adventitial fibroblasts embedded in a collagen gel. The technique is based on a depth sensing nano-indentation tester to measure the thickness and elasticity of collagen gels containing stimulated fibroblasts and a microscopy imaging system to estimate the gel area. In parallel, a simple theoretical model has been developed to calculate cell contraction force based on the measured parameters. Histamine (100 µM) was used to stimulate fibroblast contraction while the myosin light chain kinase inhibitor ML-7 (25 µM) was used to inhibit cell contraction. The collagen matrix used in the model provides a physiological environment for fibroblast contraction studies. Measurement of changes in collagen gel elasticity and thickness arising from histamine treatments provides a novel convenient technique to measure cell contraction force within a collagen matrix. This study demonstrates that histamine can elicit a significant increase in contraction force of fibroblasts embedded in collagen, while the Young''s modulus of the gel decreases due to the gel degradation.     

Creep behavior of pultruded GFRP elements – Part 2: Analytical study

This paper presents an analytical study about the viscoelastic time-dependent (creep) behavior of pultruded GFRP elements made of polyester and E-glass fibers. Experimental results reported in Part 1 are firstly used for material characterization by means of empirical and phenomenological formulations – a good general agreement is obtained using the following analytical models: (i) Findley’s power law, (ii) Bruger–Kelvin model and (iii) Prony–Dirichlet series. Based on accelerated characterization methodology – Time-Stress Superposition Principle (TSSP) coupled with Findley’s law, for a reference stress of 20% of the material ultimate stress, an elastic deformation increase of 30% is obtained after 50,000 h. The creep parameters and deformation estimated by using the Findley’s model derivations indicate a consistent prediction of time-dependent deformation and viscoelastic properties of the two types of elements analysed – laminates and beam. A straightforward formulation to predict the time-dependent elastic modulus is applied, showing that the flexural stiffness should be reduced by 25% of its initial value after 1-year and as much as 50% after 50-years. Similarly, the power law coupled to Euler’s classical beam theory suggests a reasonable adaptability to the creep phenomenon in the linear regime and proved to provide accurate predictions for deflections under flexural loading up to 40% of the ultimate strength. After 50 years, under normal service load level (1/3 of the failure load), the total creep deflection will attain almost twice the initial deflection. If taking into account the shear deformation (Timoshenko’s postulated) of the full-size element with “effective” stiffness properties such estimate is reduced nearly 25%.     

Optimal design and parameter estimation of frequency dependent viscoelastic laminated sandwich composite plates

Recent developments in optimization and parameter estimation of frequency dependent passive damping of sandwich structures with viscoelastic core are presented in this paper. A finite element model for anisotropic laminated plate structures with viscoelastic frequency dependent core and laminated anisotropic face layers has been formulated, using a mixed layerwise approach, by considering a higher order shear deformation theory (HSDT) to represent the displacement field of the viscoelastic core, and a first order shear deformation theory (FSDT) for the displacement fields of adjacent laminated face layers. The complex modulus approach is used for the viscoelastic material behaviour, and the dynamic problem is solved in the frequency domain, using viscoelastic material data for the core, assuming fractional derivative constitutive models. Constrained optimization of passive damping is conducted for the maximisation of modal loss factors, using the Feasible Arc Interior Point Algorithm (FAIPA). Identification of the frequency dependent material properties of the sandwich core is conducted by estimating the parameters that define the fractional derivative constitutive model. Optimal design and parameter estimation applications in sandwich structures are presented and discussed.     

复合材料粘弹性本构关系与热应力松弛规律研究Ⅱ:数值分析

给出了预测复合材料粘弹性松弛模量、等效热应力松弛系数和等效时变热膨胀系数的均匀化方法的有限元数值实现步骤, 研究了单向纤维复合材料随温度变化的粘弹性本构关系, 以及热应力松弛规律和热膨胀系数的时变特征。单向纤维复合材料的一维热变形分析数据显示了热应变对时间的强烈依赖关系;以数值形式给出的等效热应力松弛模量对时间的依赖关系表明, 等效的热应力松弛模量对时间的依赖性较弱, 其冲击模量和渐近模量只相差0.4 %。     

Quantifying the stress relaxation modulus of polymer thin films via thermal wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally induced instability, termed thermal wrinkling, to measure viscoelastic properties of polystyrene films as a function of geometric confinement via changes in film thickness. With application of the appropriate buckling mechanics model for incompressible and geometrically confined films, we estimate the stress-relaxation modulus of polystyrene films by measuring the time-evolved wrinkle wavelength at fixed annealing temperatures. Specifically, we use time-temperature superposition to shift the stress relaxation curves and generate a modulus master curve for polystyrene films investigated here. On the basis of this master curve, we are able to identify the rubbery plateau, terminal relaxation time, and viscous flow region as a function of annealing time and temperatures that are well-above its glass transition. Our measurement technique and analysis provide an alternative means to measure viscoelastic properties and relaxation behavior of geometrically confined polymer films.     

Micromechanical modeling and experimental characterization on viscoelastic behavior of 1–3 active composites

A study on the temperature-dependent viscoelastic behavior of (1–3 active composites) 1–3 piezocomposites and bulk piezoceramic subjected to electromechanical loading is carried out. The temperature-dependent effective properties are obtained experimentally using resonance based measurement technique. Experiments are also preformed for various fiber volume fractions of 1–3 piezocomposites subjected to constant compressive prestress and cyclic electric field at elevated temperature to understand the time-dependent behavior. Based on the measurements it is observed that the viscoelastic behavior has a significant influence on the electromechanical responses of 1–3 piezocomposites. Hence a viscoelastic based numerical model (unit cell approach) is proposed to predict the time-dependent effective properties of 1–3 piezocomposites. The evaluated effective properties are incorporated in a finite element based 3-D micromechanical model to predict the time-dependent thermo-electro-mechanical behavior of 1–3 piezocomposites and compared with the experimental observations.